The present invention relates generally to spot welding and, more particularly, to a method for single sided spot welding of hydroformed metal tubing with a metal sheet for assembling automotive structures.
Hydroformed tubular components are becoming increasingly popular in recent automotive body structural applications. During vehicle body manufacturing, such hydroformed tubular components are integrated with sheet panels, and, most economically, welding is selected for the integrating process. However, such sheet to tube welding is often limited to one-side accessibility and represents certain challenges to the welding operations. Conventional joining methods such as TIG, MIG, and Laser welding processes, which are characteristic of their one-side operation, are ideal for one-side welding. However, all these processes have at least one of several drawbacks, such as cycle time, cost, and porosity caused by zinc vapor from zinc-coated sheet. On the other hand, the robust, efficient, low-cost, and most commonly used resistance spot welding process often requires two side accessibility.
In recent years, innovative modifications were made such that spot welding can be performed from only one side. An example of such spot welding is disclosed in U.S. Pat. No. 5,708,248 to Poss et al. and U.S. Pat. No. 6,373,021 to Wang et al. However, the single side spot welding process is currently limited to non-galvanized (bare) sheet applications. Because galvanized steel cannot be welded using the single sided spot welding process, a more costly MIG welding process is used.
In single sided spot welding, because the welding electrode is applied only on one side, the other (back) side of the weld is not supported. Therefore, excessive indentation of the sheet occurs due to applied welding force and softening of sheet/tube material caused by welding heat. Such excessive indentation often leads to a xe2x80x9chollowxe2x80x9d or xe2x80x9cdoughnutxe2x80x9d shape spot weld. In addition, because of the excessive indentation, tensile stresses develop in regions around the weld. At elevated temperatures during welding, melting of materials with a low melting point such as copper from the welding electrode and zinc from the galvanized coating occurs. The presence of foreign liquid phase at elevated temperatures can readily penetrate into the grain boundaries of steel substrate under tensile stresses leading to a phenomenon so-called liquid metal embrittlement (LME) cracking. Presence of such LME cracks is not acceptable, as it will compromise weld strength and integrity.
As a result, it is desirable to provide a new method of single sided spot welding for assembling automotive structures. It is also desirable to provide a method for single sided spot welding of hydroformed metal tubing with a metal sheet for assembling automotive structures. It is further desirable to provide a method of single sided spot welding of coated metal such as galvanized steel for assembling automotive structures. Therefore, there is a need in the art to provide a method for single sided spot welding that meets these desires.
It is, therefore, one object of the present invention to provide a new method for single sided spot welding.
It is another object of the present invention to provide a method for single sided spot welding of hydroformed metal tubing with a metal sheet for assembling automotive structures.
To achieve the foregoing objects, the present invention is a method for single sided spot welding. The method includes the steps of providing a metal tube having a convex surface portion and providing a metal sheet for welding to the metal tube. The method also includes the steps of contacting the metal sheet with the convex surface portion of the metal tube and contacting the metal sheet with a welding electrode. The method further includes the steps of supplying power to the welding electrode for melting metal and solidifying the molten metal to form a weld nugget between the metal sheet and the metal tube.
One advantage of the present invention is that a method of single sided spot welding is provided for assembling automotive structures. Another advantage of the present invention is that the method incorporates a convex tube surface in areas intended for spot welding derived from a cold-working, hydroforming process which are both structurally and metallurgically more resistant to collapsing and/or indentation from the applied weld force. Yet another advantage of the present invention is that the method provides quality spot welds of complete fusion (rather than a hollow shape) free of the LME cracks and meeting a required weld button size. Still another advantage of the present invention is that the method provides a lower limit on tube thickness. A further advantage of the present invention is that the method allows material selection to be broadened by not being limited to bare metal sheet. Yet a further advantage of the present invention is that the method allows convex surfaces readily produced during the hydroforming process to be welded to a metal sheet.
Other objects, features, and advantages of the present invention will be readily appreciated, as the same becomes better understood, after reading the subsequent description taken in conjunction with the accompanying drawings.